This is MORNING EDITION from NPR News. Good morning to you. I'm David Greene.

The second global nuclear security summit wrapped up today. More than 50 international leaders were in South Korea to discuss securing vulnerable stockpiles of plutonium and highly enriched uranium. President Obama praised the effort. And he also tried to contain the damage from an off-mic comment that turned out to be on-mic. We have more from NPR's Mike Shuster in Seoul.

MIKE SHUSTER, BYLINE: The primary focus of the summit has been on terrorism - how to keep dangerous nuclear materials out of the hands of terrorists who would use them to kill innocent people. The president told the world leaders that much has been accomplished.

PRESIDENT BARACK OBAMA: We are improving security at our nuclear facilities. We are forging new partnerships. We are removing nuclear materials, and in some cases getting rid of these materials entirely. And as a result, more of the world's nuclear materials will never fall into the hands of terrorists who would gladly use them against us.

SHUSTER: One of the more unusual projects is located in Kazakhstan, at the Semipalatinsk Test Site, where the Soviet Union carried out underground nuclear tests. The facility was shut down more than a decade ago. But the U.S., Kazakhstan, and Russia are still involved in an effort to secure dangerous radioactive material that remains in its tunnels and shafts.

Scientists estimate there's enough highly irradiated material at Semipalatinsk to make a dozen weapons. As a result, the material has been entombed in special cement and sealed underground. But, the president said, the global job is not finished.

OBAMA: Of course, what's also undeniable is that the threat remains. There are still too many bad actors in search of these dangerous materials, and these dangerous materials are still vulnerable in too many places.

SHUSTER: The president also spent some time today trying to contain the damage created yesterday by an open mic. Mr. Obama was meeting with Russian President Dmitri Medvedev, and apparently unaware that the mic was hot, he was heard telling Medvedev he could use some space in this election year from the Russian leaders when it comes to the knotty problem of missile defense. This is my last election, he said. After the election, I have more flexibility.

Mr. Obama's Republican challengers immediately jumped on the comment, saying the president harbored a secret agenda and was willing to make concessions to the Russians he did not want to be made public.

The president responded today, telling reporters that arms control is complicated, that the U.S. and Russia have a special obligation to tread carefully on these matters, and frankly, he said, the current environment is not conducive to thoughtful consultations.